In the drilling of oil and gas wells by the rotary method, aqueous drilling liquids are commonly used during drilling operations. These drilling liquids are commonly called "mud". The mud is circulated down through the drill string and out the drill bit at the lower end of the drill string and then circulated up through the wellbore to the earth's surface where it is commonly recirculated back down into the wellbore. The mud contains cuttings produced in the drilling process (especially clay) and various substances that are added to give it the desired chemical and physical properties. At the surface the mud passes through solids control equipment to remove unwanted solids. The larger solids are removed by shale shakers and hydrocyclones. However, ultra-fine particles (less than about 20 microns) will continue to circulate through the system unless special solids removal equipment is used. These ultra-fine particles in water-based muds form colloidal suspensions in the water. The difficulty of removing these small particles increases as the particle size decreases.
Due to increasing environmental concerns and escalating disposal costs, there is growing incentive to reduce the volume of drilling wastes. One way of reducing the drilling waste volume is to reduce the amount of water discharged with solids disposal. Drilling rigs in some cases produce up to about two barrels of liquid waste for every foot of hole drilled. Thus, drilling operations can generate large quantities of waste.
The effectiveness of a solids control system for drilling operations can be measured by the amount of solids removed from the treated mud and the amount of water in the solids that are removed. Ideally, the solids control system will remove all drilled solids from the treated mud and the removed solids will be dry.
Several processes have been suggested to remove solids from drilling muds. For removing particle sizes ranging down to about 20 microns in diameter, conventional decanting centrifuges are used. To remove solids below about 20 microns, coagulants and flocculating agents have been added to muds. These so-called chemically enhanced centrifugation processes aggregate the fine, drilled particles allowing them to be more easily separated from the fluid by centrifugation. Coagulants that have been used include aluminum, iron and calcium salts, and cationic polymers. Flocculants that have been used include water-soluble polyacrylamides, water-soluble poly(acrylic acid) and its homologues, water-soluble poly(ethylene oxide) resins, polyvinyl alcohol, water-soluble carboxymethylcellulose, and guar gum. The water recovered by centrifugation is returned as needed to the mud system and the separated solids are discharged as waste.
Although chemically enhanced centrifugation processes are effective in removing solids from a drilling fluids system, there is a substantially unfilled need for an improved process for dewatering aqueous drilling fluids without adding coagulants and flocculants. Introducing chemically active substances to a drilling fluids system can affect the drilling fluids physical and chemical properties. In addition, chemically enhanced centrifugation can also remove large amounts of liquid with the solids.